Part of the reason is that they have a better shot at keeping the script secretive. The key to 24 is the shock value.
I can't argue with that. If you don't have a master plan, there is very little chance of it leaking out.
Remember season 2? The nuclear bomb? What happened after the bomb exploded? Does anyone remember?
The show peaked too soon and the writers had no idea how to finish the year out. It was all anticlimactic. Planning broad story arcs beforehand would have prevented this.
SD